The present invention concerns an apparatus for monitoring the level of liquid in a tank or reservoir and constitutes an improvement in the apparatus of U.S. Pat. No. 4,163,391 issued Aug. 7, 1979.
Apparatuses are already known, of the type comprising a probe formed by a resistive metal element having a high temperature coefficient, which is intended to be immersed in the liquid in a tank or reservoir, a means for supplying the probe with current, and means for comparing the initial voltage Uo at the terminals of the probe, to the voltage Ut at the terminals of the probe after a period of time t, at which the probe has attained a condition of thermal stability. The expression initial voltage Uo is used to denote the voltage at the terminals of the probe at the moment to at which the current supply means is set in operation. It is known in fact that a resistive metal element having a high temperature coefficient undergoes an increase in its own resistance in dependence on temperature, and consequently the current which passes therethrough, it being appreciated that those two parameters are linked. Thus, when such a resistive element is immersed in a liquid, its own resistance depends on the level of liquid in which it is immersed, since the part of the resistive element which is above the liquid and which is therefore exposed to the air is cooled to a much lesser degree than the immersed part.
When the level of liquid in which the resistive element is immersed rises, the mean temperature of the element falls, and therefore accordingly also its resistance.
It will be appreciated that the opposite phenomenon occurs when there is a fall in the level of liquid in which the resistive element is immersed.
The aforementioned U.S. Pat. No. 4,163,391 thus describes various embodiments of apparatuses for monitoring the level of liquid in a tank or reservoir, making use of such a principle.